Controlled release hydroxybutyrate polymer microspheres

ABSTRACT

Controlled release microspheres of hydroxybutyrate polymer comprise active ingredient and hydroxybutyrate/hydrovalerate copolymer. The microspheres comprise a skin which is distinct from the general bulk of the microspheres, and this skin may have a porosity covering from 0-50% of the total surface area of the microsphere, this porosity being largely controlled by varying the hydroxyvalerate content of the polymer. The porosity of the interior of the microspheres can also be regulated. Control of the two porosities permits the attainment of a wide range of release rates for a wide range of active ingredients. The microspheres can be used in a wide variety of pharmaceutical, veterinary and agricultural applications.

This invention relates to the microencapsulation of active substancesand to microspheres thus prepared.

The microencapsulation of active substances, that is, substances whichhave a chemical or biological effect in a suitable environment, is atechnique which is well known to the art. Typical examples of activesubstances which have been encapsulated are drugs and fungicides. Thepossible reasons for microencapsulation are numerous, for example, topermit the administration of an active substance which could nototherwise be administered, to preserve the active substance in a hostileenvironment until it can be released in the correct environment, or toextend the release of an active substance over a period of time so as toextend correspondingly the effect of the active substance. The twolast-named categories are often described as "controlled release"applications. Controlled release is an especially useful property infields such as pharmaceuticals where use has been made of gelatincapsules. A more recent development is the use of microspheres,especially those prepared by solvent evaporation. In this case, theactive substance and the microsphere- forming material are dissolved ordispersed in a liquid, commonly an organic liquid, which is thendispersed in a liquid in which the organic liquid is immiscible (wateris commonly used) and the organic liquid removed by evaporation to leavemicrospheres.

The procedure is described by Bissery and co-workers in a paper given atthe 3rd Exposition-Congress of the International Technology of Pharmacy(Assoc. Pharm. Galenique Ind. 1983), Vol. 3, pp. 233-9. This paperdescribes the application of the technique to a number of polymers,including poly(β-hydroxybutyrate). This polymer is described in afurther publication by Bissery (chapter 4 of "Microspheres and DrugTherapy : Pharmaceutical, Immunological and Medical Aspects", ed. Davis,Illum, McVie and Tomlinson, Elsevier 1984, pp 217-227) in connectionwith the anticancer agent CCNU.

Bissery has observed that the surface of a poly(β-hydroxybutyrate)(hereinafter referred to as "PHB"; microsphere contains many macroscopicsurface pores. This undoubtedly contributes to the fact that PHBmicrospheres release the active substances encapsulated therein veryquickly; the release rates given by Bissery in the second publicationreferred to hereinabove are very high. However, this speed of release isnot suitable for the whole range of active substances and this limitsthe usefulness of PHB.

It has now been found that it is possible to prepare microspherescomprising PHB wherein the release rate may be altered in a predictablemanner. There is therefore provided, according to the present invention,controlled release hydroxybutyrate polymer microspheres comprising atleast one active substance, the polymer comprising a copolymer of3-hydroxybutyric acid and 3-hydroxyvaleric acid, and the microspherescomprising a continuous skin covering the surface thereof.

PHB, sometimes known in the literature as poly(3-hydroxybutyric acid),is a thermoplastic polymer which is accumulated by many organisms.European Patent Specifications 15669 and 46344 describe typicalprocesses for making PHB. The production ofhydroxybutyrate/hydroxyvalerate copolymers is described by, for example,Wallen and co-workers in "Environmental Science and Technology" 8(1974), 576-9.

It is a special feature of the microspheres of this invention that, incontrast to the microspheres of the known art, they have a skin, thatis, a thin surface layer different in structure to that of the bulk ofthe microspheres. This skin is continuous over essentially the entiresurface of the microspheres. It is, however, permissible that it mayhave pores therein which give rise to openings at the surface of theskin. The skin can in fact vary between essentially completely pore-lessand quite highly porous (pore ends covering up to about 50% of the totalskin surface area). It is the combination of the nature of the skin andthat of the interior of the microsphere and the susceptibility of theseto controllable alteration which give the microspheres of the presentinvention unique advantages over the known art.

The controlled release characteristics of the microspheres according tothe present invention can be modified either by controlling the natureof the skin or by controlling the nature of the interior of themicrospheres. The nature of the skin may be altered by varying thehydroxyvalerate content of the polymer. This can be done either byvarying the hydroxyvalerate content of the copolymer itself or byblending with the copolymer a proportion of PHB homopolymer. It has beenobserved that as the valerate content rises, the porosity of the skindecreases. At high valerate contents, there is no skin porosity at all,and active ingredients can escape only by diffusion through the polymermatrix.

Other factors such as temperature of evaporation of the solvent, type ofsolvent and polymer concentration in the organic phase have some effecton skin porosity and thickness, but they exert an effect which isrelatively minor in comparison to that produced by hydroxyvaleratecontent variation.

The interior of the microspheres may be modified in a number of ways,but they all have the common factor of varying the internal porosity ofthe microspheres. One way of doing this is to add amphipathic materialto the copolymer. It has been observed that microspheres produced fromthe pure copolymer are solid, essentially non-porous spheres. However,the addition of amphipathic material, that is, material having affinityfor both phases of a two phase oil/water system, increases the porosity.The amphipathic material may be, for example, a commercial surfactant ora number of such surfactants. Copolymer from some sources has been foundto contain already an amphipathic natural material the nature of whichis presently unknown but which can be utilised in place of an additionof amphipathic material. Thus, in order to reduce the porosity incopolymers of this type, the copolymers are purified to the requireddegree.

A further method of modifying the interior is by using a double emulsiontechnique. This is a well-known technique which involves the stableemulsification of a first liquid in a second liquid with which the firstliquid is not compatible, followed by the stable emulsification of theresulting emulsion in a third liquid with which the second liquid is notcompatible. Commonly, the first and third liquids are aqueous and thesecond liquid is a polymer or polymer precursor, such that the endresult is a dispersion of polymer microspheres which comprise an innerstructure, this generally comprising discrete cells or continuousporosity, depending on the nature of the polymer. In the presentinvention, the emulsification of an aqueous liquid into a solution ofcopolymer in a suitable solvent can readily give a desired degree ofporosity. The internal aqueous phase may be stabilised by any of themeans known to the art, for example, surfactants. It is a feature ofsome copolymers containing amphipathic natural material of the typedescribed hereinabove that stabilisation of the internal aqueous phaseis achieved by this means alone, without further addition of surfactant.

The active substance for use in this invention can be any suitableactive substance. It can be, for example, a pharmaceutical, a herbicide,an insecticide or a fungicide. Because of the ability (hereinabovedescribed) to alter the surface morphology and therefore the releasecharacteristics of the microspheres according to this invention, anunusually wide range of active substances can be accommodated. It is ofcourse permissible to incorporate more than one active substance in asingle microsphere. The active ingredient may be incorporated directlyinto the copolymer, or in solution or dispersion form. It may be withinthe cells or pores, the polymer matrix or both.

The microspheres of this invention may be prepared by any means known tothe art suitable for the preparation of such microspheres of suitableskin and internal structure. For example, they may be prepared by thesolvent evaporation method described in the Bissery publicationsreferred to hereinabove. An alternative method is the double emulsionmethod hereinabove described. A third method is that described in acopending Australian patent applicant by the same applicant, wherein acopolymer solution is added to a continuous phase which contains theactive ingredient.

The invention is further illustrated by the following examples in whichall parts are expressed by weight.

EXAMPLE 1 Preparation of microspheres containing a pesticide

1 part of a hydroxybutyrate/hydroxy- valerate copolymer (19% valeratecontent by weight) and 0.25 parts of "Chlorpyrifos" (trade mark)pesticide were dissolved in 30 parts chloroform. This mixture was addedto a stirred, heated (55° C.) aqueous solution (0.27%) of an 82%hydrolysed grade of polyvinyl acetate a 4% aqueous solution of which hada viscosity of 9.0 cps at 20° C. The emulsion thus formed was stirred at250 rpm and maintained at 55° C. for 3 hours until the chloroform hadcompletely evaporated. Washing, filtering and drying gave whitemicrospheres of diameter of from 100-500 um.

Examination by scanning electron microscope at magnification × 700revealed an essentially pore-free skin and sectioned microspheresexhibited a highly porous interior (approximately 40% by volume ofspherical pores, the mean pore diameter being about 15 um). X-rayfluorescence revealed the presence of the pesticide within the polymermatrix.

EXAMPLE 2 An example which demonstrates the effect of raising thehydroxyvalerate content of the copolymer

Example 1 was repeated, with the substitution of an identical quantityof a copolymer containing 27% hydroxyvalerate by weight. The interior ofthe microspheres prepared from this copolymer comprised about 70% byvolume of pores with a mean pore size of about 5 um. The skin on themicrospheres was essentially pore-free.

EXAMPLE 3 An example which demonstrates the effect of the removal ofnaturally-occurring amphipathic material

Example 1 was repeated, with the additional step that the copolymer waspurified prior to microsphere manufacture by twice precipitating it fromchloroform solution with methanol.

The resultant microspheres were smooth-skinned (no pores) and had aporosity of 15%, the pores being spherical and of mean size 2 um. Theclaims defining the invention are as follows.

We claim:
 1. Controlled release hydroxybutyrate polymer microspherescomprising an amphipathic material and at least one active substanceselected from pharmaceuticals, herbicides, pesticides, fungicides, andmixtures thereof, the hydroxybutyrate polymer comprising a co-polymer of3-hydroxybutyric acid and 3-hydroxyvaleric acid, and the microspherescomprising a continuous skin covering the surface thereof wherein theskin comprises pores whose total surface area expressed as a percentageof the total surface area of the microspheres is up to 50% maximum.
 2. Aprocess of administering an active ingredient at a desired locus bymeans of controlled release hydroxybutyrate polymer microspheresaccording to claim
 1. 3. Controlled release hydroxybutyrate polymermicrospheres according to claim 1, wherein the interior of themicrospheres beneath the skin comprises porosity.
 4. A process ofpreparing controlled release hydroxybutyrate polymer microspheresaccording to claim 1, wherein the surface porosity is selected by theselection of the proportion of hydroxyvalerate content of thehydroxybutyrate polymer.
 5. A process according to claim 4, wherein theproportion of hydroxyvalerate in the hydroxybutyrate polymer is alteredby altering the proportion thereof in thehydroxyvalerate/hydroxybutyrate copolymer.
 6. A process according toclaim 4, wherein the proportion of hydroxyvalerate in thehydroxybutyrate polymer is altered by the addition thereto of PHBhomopolymer.
 7. A process of preparing controlled release claim 3,wherein the porosity of the interior of the microspheres is determinedby the inclusion in the polymer of amphipathic material.
 8. A processaccording to claim 7, wherein the amphipathic material comprises atleast one surfactant.
 9. A process according to claim 7, wherein theporosity is determined by controlling the level of naturally-occurringamphipathic material in the polymer.
 10. A process of preparingcontrolled release hydroxybutyrate polymer microspheres according toclaim 3, wherein the porosity of the interior of the microspheres isdetermined by the use of a double emulsion process.